Certain polymeric films have been found to exhibit pyroelectric properties, i.e., to exhibit electrical polarization reversibly with a change in temperature of the material, with the amount of polarization being directly proportional to the temperature change, and piezoelectric properties, i.e., electric polarization as a response to mechanical strain, with the amount of polarization being directly proportional to the mechanical strain applied. To form a pyroelectric and piezoelectric element from these polymer films, same are typically exposed to an intense electric field (a process known as "poling").
In some instances, the foregoing element has also been termed an "electret". For example, U.S. Pat. Nos. 3,794,986 and 3,833,503 discuss this concept of an electret in the broad sense as being any element capable of retaining an electric polarization, whether this polarization is on the outside or the inside of the polymeric material. The term "electret" is also utilized in a more narrow sense as a substance wherein an electric field is formed on the outside of the material only. Thus, in the narrow sense the term is characterized by the display of a surface charge density, as is discussed in U.S. Pat. No. 3,607,754, together with the foregoing references. An electret in the narrow sense, i.e., that displaying a surface charge density, is not capable of providing stable pyroelectric and piezoelectric characteristics. As foregoing U.S. Pat. No. 3,833,503 discusses, in order to achieve a pyroelectric element having stable pyroelectricity, the macroscopic polarization by the space charges in the electret must be depolarized. The reference goes on to indicate that in order to achieve stable pyroelectric characteristics, the space charges, or the surface charge on the conventional electret in the narrow sense discussed above, must be first removed. This macroscopic polarization is depolarized, as taught in this reference, by short circuiting the opposite sides of the electret for a sufficient time at an elevated temperature or for a longer time if ambient temperatures are utilized. Furthermore, as discussed in U.S. Pat. No. 3,794,986, when the temperature of such an electret in the narrow sense is elevated, reproduceable, i.e., stable, pyroelectricity is not obtained.
Therefore, to form an element having stable pyroelectric and piezoelectric characteristics, most of the surface charge introduced during the poling process is typically drained from the element by shorting out the electrodes on both sides of the film. Such a procedure would, of course, destroy any electret properties in the narrow sense as discussed above.
To summarize, the word "electret" as commonly used refers to an electret in the narrow sense, i.e., a material which exhibits a permanent surface charge density. An electret in this narrow sense cannot be equated to a stable pyroelectric and piezoelectric element.
One film which has been found capable of providing stable pyroelectric and piezoelectric elements is comprised of polyvinylidene fluoride. It is widely known in the literature that the formation of an element displaying stable pyroelectric and piezoelectric properties from polyvinylidene fluoride requires that a film of the appropriate crystalline structure (preferably beta crystallinity, although gamma crystallinity is acceptable) must be prepared prior to the poling operation. Polyvinylidene fluoride, when manufactured in film form, such as by an extruder, etc., has a crystalline structure which is of the alpha type. It has been found, however, that the alpha crystalline form is not capable of being poled to provide a useful stable pyroelectric and piezoelectric element. This is true because these characteristics arise directly from the presence of specific types of symmetry in the crystals within the structure. Beta and gamma crystallites possess the appropriate symmetry while alpha crystallites do not.
In order to produce a film capable of being poled to produce the requisite and desired stable electrical characteristics, the films are typically converted to primarily the beta type of crystallinity conventionally by uniaxial or biaxial orientation, i.e., stretching the film in a heated state and retaining same in that stretched state while the film is allowed to cool. The resultant film exhibits beta crystallinity to varying degrees depending on the amount of stretching.
In some cases, this mechanical orientation can be applied simultaneously with the poling process, as is taught in U.S. Pat. No. 4,308,370, so as to form the beta crystallites by stretching and inducing polarization in the same step.
While such mechanical orientation techniques are effective for the production of films which can be poled and thus transformed into stable pyroelectric and piezoelectric elements, the necessity for orientation has several disadvantages: first, such orientation is a costly processing step; second, during orientation an anisotropic film is created which results in anisotropic piezoelectric properties. (Since the most highly piezoelectric elements usually result from highly uniaxially oriented films, the anisotropy can be considerable); third, stable pyroelectric and piezoelectric coatings cannot typically be applied directly onto an article, as by solvent casting, for example, because the coating cannot typically be oriented once placed on the article.
One approach to solving the aforementioned problems is disclosed in U.K. Pat. No. 1,349,860. Therein is taught the concept of use of a vinylidene fluoride copolymer to achieve a stable pyroelectric and piezoelectric element. The addition of a comonomer to the vinylidene fluoride monomer during the polymerization sequence is taught to provide a film apparently containing the requisite crystalline structure for subsequent poling to provide stable pyroelectric and piezoelectric characteristics thereto. With this system, somewhat lengthy polarization times, i.e., about 2 hours, are disclosed. While the reference teaches that shorter times can be used, lengthier times are taught to be preferred.
We have discovered that if the polyvinylidene fluoride polymer can be maintained in an amorphous state prior to poling, the poling process by itself results in an element displaying stable pyroelectric and piezoelectric properties. Apparently, the poling itself creates the desired .beta.-crystallinity in the film or coating, with no necessity for mechanical orientation.
The prevention of crystalline formation is achieved by utilizing a polymeric blend, with one of the polymers being polyvinylidene fluoride and the other polymer being one that is miscible therewith.
By keeping the miscible polymer blend in an amorphous state until poling is undertaken, beta crystallinity, and thus the desired electrical characteristics are achieved solely by the action of the polarization. Such a technique cannot be applied to pure polyvinylidene fluoride because the pure polymer has such a high tendency to crystallize that crystallization cannot be sufficiently prevented.
The function of the miscible polymeric blend component is therefore to reduce the tendency of the polyvinylidene fluoride to crystallize and thus to form a substantially amorphous film prior to poling.
The use of polymer blends of polyvinylidene fluoride with miscible polymers is itself known in the art. Such polymers include acrylates, e.g., polymethyl acrylate; methacrylates, e.g., polymethyl methacrylate and polyethyl methacrylate; polyvinyl acetate; poly N,-N-dimethyl acrylamide; polyvinyl methyl ketone and poly N-vinyl-2-pyrolidone. Such blends have in fact been mechanically oriented to produce the beta crystalline form of the polyvinylidene fluoride within the blend composition and subsequently poled. One would expect that the desired electrical properties would be less than those of pure polyvinylidene fluoride because less polyvinylidene fluoride is present in the composition, and such experiments have in fact been reported with the expected results. (Lee, H.; Salomon, R. E.; and Laebs, M. N.; Macromolecules, 11, p. 171, 1978.)
We have now found that by using a simple blend of polymers, processing same to a film or a coating on an article can be undertaken without the necessity of mechanical orientation of the film, yet a film or coating displaying stable pyroelectric and isotropic piezoelectric characteristics can be obtained.